1. Field of the Invention
This invention relates to liquid developing apparatus used to deposit electroscopic toner particles dispersed in a fluid onto the surface of a moving recording member in accordance with a latent electrostatic image formed thereon, and more particularly to such an apparatus which applies the liquid developer only to one surface of the recording member.
2. Description of the Art
Several techniques are known for applying electroscopic toner to latent electrostatic image bearing record members so that toner will adhere to the record member in desired areas to develop the image. One technique is by cascading the particulate toner in dry form onto the surface of the record member, and removing the excess toner either by allowing it to slide off the record member by the force of gravity, or by blowing the excess toner with a fan. Another technique is to entrain the toner particles in a finely divided magnetic powder and to use a magnetic brush for distributing the toner over the surface of the record member. A third technique for distributing the toner is to entrain the toner in a dielectric liquid which is then brought in contact with the surface bearing the latent image. The developer apparatus of the present invention utilizes the latter technique.
A significant problem encountered in developer apparatus that uses liquid entrained toner is commonly known as "boundary layer depletion". This may occur even after a very short period of use, since the concentration of toner in the liquid near where the liquid is brought into contact with the recording member may lower very rapidly when developing a "dark area" of the latent image.
Other than the total emersion of the recording member in the liquid entrained toner for the purpose of development, which was not very effective with flat record members, there has existed the problem of supplying an adequate concentration of toner to ensure complete development. One of the earliest attempts to solve this problem was to use a roller dipped into a liquid toner bath. The roller, which was also conducting, was then rotated to provide a sufficient turbulent action to the liquid to provide a constant changing and replenishing action to the liquid entrained toner at the processing nip. Such an apparatus is illustrated in U.S. Pat. No. 3,367,791, entitled LIQUID DEVELOPMENT OF ELECTROSTATIC IMAGES.
As processing speeds increased, equipment modifications were required to keep pace with the increased through-put. It was found that as the recording member was passed over a rotating developing electrode, the developing zone was relatively small and that effective development was limited by the volume of liquid dispersed toner that the rotating electrode could deliver to the processing nip. By placing a plurality of rotating electrodes in tandem, the volume of toner liquid could be substantially increased so that the speed at which the recording member moves could be increased. The image is fully developed gradually by building up small amounts of toner particles supplied by the successive rotating electrodes. An example of developing apparatus of this type may be found in U.S. Pat. No. 3,774,574. However, such a development apparatus would occupy substantially more space than apparatus that used single rotating electrodes, especially in a color apparatus that would require as many as four independent developing applicator sections for three colors and a fourth for black or neutral density.
With the advent and popularity of color images produced by electrophotographic means there resulted in an increase in the minimum distance between the recording member and the applicator which may be termed the "image gap", which consists of the total thickness of all the multi-step toners that are capable of being applied to a recording member to create a composite color image. Thus when a large image gap is involved, the thin layer of liquid dispersed toner that adheres to a cylindrical development electrode would not make contact with each of the images to be developed. Such a smooth-surfaced applicator is capable of supporting a liquid developer meniscus of about 0.015" thickness due to the low viscosity of conventional liquid developers and the smooth nature of the roller. When a large film-to-roller spacing is required e.g. 0.050", the liquid developer would not make contact with the film.
U.S. Pat. No. 3,561,400 relates to an improved apparatus that attempted to solve many of the problems mentioned above. Generally, the apparatus comprises a shroud that surrounds a cylindrical roller. The shroud has an opening therein which permits a portion of the roller to project beyond the shroud. A pump provides a supply of liquid entrained toner into the space between the roller and the shroud. The roller is rotated to carry a film of the developer liquid over the projecting portion of the roller to contact the record bearing members.
Thus, it can be seen that any attempt to increase the through-put of apparatus that produces color images requiring a large image gap would result in drastically increasing the diameter of the rotating electrode to obtain an adequate electric field as well as the proper developing zone.